The present invention relates to an image forming apparatus capable of protecting a toner image from contamination and an image forming unit therefor.
A laser printer, facsimile apparatus, digital copier or similar electrophotographic image forming apparatus forms a latent image on a photoconductive element or image carrier, develops it to produce a toner image, and transfers the toner image to a paper or similar recording medium. At the time of image transfer, a bias opposite in polarity to the charge of toner forming the toner image is applied from the paper side in order to electrostatically deposit the toner on the paper.
However, the problem with the above bias scheme is that if the paper is charged by friction acting between it and conveying members, the transfer of the toner from the photoconductive element to the paper is promoted more than expected. As a result, the toner is apt to scatter and deposit on the background of the paper, lowering image quality.
Japanese Patent Laid-Open Publication No. 6-118803, for example, discloses an arrangement for preventing the toner from scattering during image transfer. The arrangement taught in this document includes a varistor or constant voltage element connected to ground and located upstream of an image transfer station in the direction in which a paper moves toward the image transfer station. The varistor may be replaced with a discharging member for applying a DC current opposite in polarity to a bias for image transfer. The discharging member discharges a paper when the paper is brought into contact therewith.
A developing device included in an image forming apparatus of the type described induces the scattering of toner toward a paper. In light of this, there has been proposed a conductive sheet member located downstream of a position where a photoconductive element and a developing roller included in the developing device face each other in the direction of movement of the element. A bias voltage of the same polarity as a bias voltage for development is applied to the conductive sheet member via a terminal member. The terminal member is implemented by a leaf spring and anchored at opposite ends thereof to the shaft of the developing roller and the conductive sheet member. In this configuration, the sheet member intercepts toner flying from the developing device toward an image transfer station adjoining the developing device. In addition, the bias voltage prevents such toner from depositing on the sheet member.
However, if a paper contacts the above conductive sheet member applied with the bias of the same polarity as the toner, the paper is charged by the sheet member. As a result, it is likely that the paper is unevenly charged or repulses the toner, resulting in irregular image transfer or toner scattering which would bring about a defective image.
Technologies relating to the present invention are also disclosed in, e.g., Japanese Patent Laid-Open Publication Nos. 5-40392, 9-222790 and 11-174751.